This invention relates to an improvement of the hot isostatic pressing method (this method is hereinafter referred to as the HIP method), a remarkable development of recent years, wherein the improvement comprises a new HIP method which employs a unique capsule for sintering.
In general, hot pressing methods can be divided into two main classes; namely, the normal hot pressing method and the hot isostatic pressing method.
The hot pressing method is one of the effective methods for sintering materials, thus producing a sintered body of high density.
In the above method, however, since the material is pressed in only one direction, the particles of the sintered body tend to be arranged in the same direction. Namely, when the sintering is completed, the sintered body shows differing mechanical and physical properties in the vertical plane and in the horizontal plane thereof relative to the pressing direction.
As a sintering method which removes the above anisotropy, the HIP method has been developed wherein the material is uniformly compressed making use of a gaseous body(mainly inert gas) as a pressure medium, while the material is heated.
The conventional HIP method is conducted in such a way that powder is loaded into capsules of sheet metal standing on a hydraulic vibrating table which can be lowered. The capsules are then sealed by welding covers to their open ends. The particles have a very small contact area and therefore the powder has extremely poor thermal conductivity. Therefore it is cold-compacted at a range of from 400 to 1,000 kg/cm.sup.2. After compacting, the capsules are evacuated in a conventional furnace at 400.degree.-500.degree. C, closed and heated to the required final temperature.
Furthermore, when the powder-like material, which, in general, contains large amounts of absorbed gas, is charged into the capsule made of metal plates and degassed, and subsequently the capsule is sealed in an air-tight manner, due to heating during the HIP operation, gas adhering to the power-like material is dispelled from the material and remains within the capsule, ill-affecting the various properties of a sintered body.
Reviewing the above disadvantages of the conventional HIP method, a method which undertakes a pre-operation before conducting the HIP operation can be considered wherein the pre-operation comprises shaping the powder-like material into the desired form, and subsequently sintering the material by the ordinary method or by the hot pressing method whereby gas adhering to the powder-like material is expelled from the material and thus produces a sintered body almost entirely free from apertures or holes as heretofore.
In general, the size of the particles which form a sintered body is influenced by the sintering temperature such that the size increases corresponding to the elevation of the sintering temperature. However, a sintered body demonstrates greater strength when the component particles are sintered finely and densely.
In the production of ceramics, which require high density thereof, it is desirable that the material be sintered at a low temperature so that the grain growth of component is restricted.
However, as far as the conventional sintering method is concerned, when the material is sintered at a low temperature, the sintered body may still contain a considerable amount of pores therein although the grain growth of component particles can be supressed and gas adhering to the powder-like material can be removed.
It is impossible to directly sinter the porous sintered body by the HIP method, since high pressure gas infiltrates to the inside of the sintered body.
Accordingly, a method for increasing the density of a sintered body which comprises coating the surface of the porous pre-sintered body with glass and subsequently compressing the coated presintered body by means of high pressure gas has been developed.
However this method, which utilizes glass, is restricted in its use thereof in view of the components of the sintered body and the glass material. Namely, alkaline earth metal or SiO.sub.2, which is contained within glass, reacts with the sintered body, greatly influencing the properties of the sintered body. Furthermore, glass melts during heating and flows downward whereby the glass coating layer can not be kept at a uniform thickness over the entire surface of the sintered body. As has been described heretofore, the selection of the capsule which coats the sintered body is of great significance in making full use of the HIP method.
It is therefore an object of the present invention to provide an HIP method for producing a sintered body of high density which is characterized in its use of a desired metal powder such as titanium as the material for the capsule.
It is another object of the present invention to provide a capsule used in the above HIP method wherein the capsule which is composed of pure titanium and has a uniform thickness of more than 8 micron is formed over the entire surface of the pre-sintered material.
It is still another object of the present invention to provide a method for forming a pure titanium capsule which is uniform in thickness over the entire surface of a pre-sintered material.
The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims.
The invention, itself, however, and additional objects and advantages thereof will best be understood from the following description of embodiments thereof when read in connection with the accompanying drawings and photographs.